Epithelial mucosal surfaces are lined with fluids whose volume and composition are precisely controlled. In the airways, a thin film of airway surface liquid helps protect mammalian airways from infection by acting as a lubricant for efficient mucus clearance (Hobbs et al., J. Physiol. 591: 4377 (2013), Knowles et al., J. Clin. Invest. 109:571 (2002)). This layer moves cephalad during mucus clearance and excess liquid that accumulates as two airways converge is eliminated by Na+-modulated airway surface liquid absorption with Na+ passing through the epithelial Na+ channel (ENaC) (Hobbs et al., J. Physiol. 591: 4377 (2013), Knowles et al., J. Clin. Invest. 109:571 (2002)). Critically, the mechanism by which ENaC activity is regulated in the airways is poorly understood. Recently, evidence has been accumulating that molecular regulators in the airway surface liquid can serve as volume sensing signals whose dilution or concentration can alter specific cell surface receptors that control ion transport rates to either absorb or secrete airway surface liquid as needed (Chambers et al., Respir. Physiol. Neurobiol. 159:256 (2007)). As one of the regulated targets, ENaC must be cleaved by intracellular furin-type proteases and/or extracellular channel activating proteases (CAPs) such as prostasin to be active and to conduct Na+ (Planes et al., Curr. Top. Dev. Biol. 78:23 (2007); Rossier, Proc. Am. Thorac. Soc. 1:4 (2004); Vallet et al., Nature 389:607 (1997); Chraibi et al., J. Gen. Physiol. 111:127 (1998)). ENaC can also be cleaved and activated by exogenous serine proteases such as trypsin, an action that is attenuated by the protease inhibitor aprotinin (Vallet et al., Nature 389:607 (1997)). When human bronchial epithelial cultures are mounted in Ussing chambers where native airway surface liquid is washed away, ENaC is predominantly active, suggesting that cell attached proteases are predominant (Bridges et al., Am. J. Physiol. Lung Cell. Mol. Physiol. 281:L16 (2001); Donaldson et al., J. Biol. Chem. 277:8338 (2002)). In contrast, under thin film conditions, where native airway surface liquid is present, ENaC activity is reduced, suggesting that airway surface liquid contains soluble protease inhibitors (Myerburg et al., J. Biol. Chem. 281:27942 (2006); Tarran et al., J. Gen. Physiol. 127:591 (2006); Gaillard et al., 2010 Pfleugers Arch, 460: 1-17).
Recently, it has been shown that the Short Palate Lung and Nasal epithelial Clone (SPLUNC1) protein comprises up to 10% of the total protein in the airway surface liquid and can readily be detected in both nasal lavage and tracheal secretions (Bingle, C. D., and Craven, C. J. (2002) PLUNC: a novel family of candidate host defense proteins expressed in the upper airways and nasopharynx Hum Mol Genet 11, 937; Campos, M. A., et al. (2004) Purification and characterization of PLUNC from human tracheobronchial secretions Am J Respir Cell Mol Biol 30, 184; Lindahl, M., Stahlbom, B., and Tagesson, C. (2001); Identification of a new potential airway irritation marker, palate lung nasal epithelial clone protein, in human nasal lavage fluid with two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight Electrophoresis 22, 1795). SPLUNC1 appears to be a volume sensing molecule since it can be secreted onto the mucosal surface of the superficial epithelia where ENaC is expressed (Bartlett et al., J. Leukoc. Biol. 83:1201 (2008); Bingle et al., J. Pathol. 205:491 (2005)). Furthermore, SPLUNC1 has been demonstrated to contain a subdomain that functions as an inhibitor of ENaC through its N-terminal domain.
The present invention discloses novel specialized non-naturally occurring peptides that mimic the properties of SPLUNC1 in regulation of sodium channels by binding to and inhibiting ion transport to regulate sodium absorption and fluid volume and treat disorders responsive to modulating sodium absorption.